It is known that the dermis has a high density of capillary blood vessels when compared with an epidermis or a subcutis and also a lymph vessel end exists therein, so that in particular, a medical agent injected thereto directly is shifted to a blood vessel or a lymph vessel and an absorption speed thereof being absorbed in the body fluid is speedy. In particular, it is possible in the dermis to make a medical agent using a macromolecular substance such as hormone, antibody drug, cytokine and the like to be absorbed into the blood efficiently. Also, it is known that the dermis is a place of efficient immunity and it is possible to make a saving of applied dose of vaccine or to strengthen sensitization of a weak vaccine.
Also, it is known for adult human beings that the dermis exists approximately at a certain depth from the body surface (surface of stratum corneum). In other words, this fact means in case of injecting a medical agent into the dermis that it is possible to use a puncture needle having the same length (depth) with respect to these human beings.
Generally, the width of the dermis D is around 1 mm to 4 mm (average value is 1 mm to 2 mm) if the vertical direction with respect to the body surface F is made to be a reference and also, as shown in FIG. 10, the dermis D exists in the skin so as to be sandwiched between an epidermis E which includes a stratum corneum SC and has width of around 0.06 mm to 0.1 mm and a subcutis S.
Accordingly, it is difficult to insert a tip portion of the puncture device, for example, a needlepoint of the puncture needle accurately to the dermis which exists between the epidermis and the subcutis and if the needlepoint is inserted erroneously to the subcutis or the like, there occurs a problem such that a medical agent cannot be absorbed efficiently.
In recent years, for example, it is attempted that the macromolecular medicine mentioned above is administered continually or by one-shot into the dermis as a target and in such a case, in particular, the above-mentioned problem becomes conspicuous.
Here, a hypodermic injection device is known in which the length of the puncture needle to be inserted into a living body is defined in order to inject a medical agent to the dermis in the body (JP2007-137343). In addition, a medicinal solution injection device is also known in which the depth (insertion depth) of the puncture needle to be inserted into the skin is defined to be a predetermined length in order to inject a medical agent into a specific layer which exists in the skin and the puncture needle is inserted into the skin from the vertical direction with respect to the body surface (JP2005-087519).